1. Field of the Invention
The present invention relates to a noise filtering circuit including a plurality of delay circuits connected to each other in series and filtering low-level noise included in a high-level input signal and high-level noise included in a low-level input signal in each of the plurality of delay circuits connected to each other in series to be able to effectively filter two types of noise signals, and an operating method thereof.
2. Description of the Related Art
A technology of filtering noises included in an input signal is essential to secure stability of the overall circuit and obtain a required output signal and has been widely applied to various types of circuits. In particular, a technology of filtering a high-level noise signal overlapping a low-level input signal and a low-level noise signal overlapping a high-level input signal, to be separate from a normal input signal, is very important to obtain a stable output signal.
A noise filter circuit according to the related art branches one input signal to generate two input signals having different phases and transmits the generated steady-phase input signal and a negative phase input signal, respectively, to independent delay circuits to carry out turn-on filtering and turn-off filtering. The noise filter circuit may filter high-level noise included in a low-level input signal in the turn-on filtering operation and low-level noise included in a high-level input signal in the turn-off filtering operation.
The result signal of the turn-on filtering and the result signal of the turn-off filtering are input to a flip flop such as an RS latch. Consequently, a separate flip flop is necessarily connected to a later stage of the turn-on filtering circuit and the turn-off filtering circuit, such that the overall circuit configuration may be complicated and costs may be increased accordingly.
The Related Art Document below relates to a semiconductor memory device and a control method thereof and includes a buffer receiving an input signal, a noise filter circuit, and a latch to carry out a turn-on filtering. However, like the related art introduced earlier, the Related Art Document below requires a latch essentially connected to a later stage of the turn-on filtering circuit, and therefore does not disclose a noise filter circuit capable of carrying out both of the turn-on filtering and the turn-off filtering without the latch.